An electronically slip-controllable braking system is described in German Patent Application No. DE 10 2009 001135 A1.
This braking system encompasses an electronically controllable first actuator suite, referred to below as a “primary” actuator suite, in the form of a conventional ABS/ESP brake system for individual-wheel modulation of brake pressure as a function of the slip conditions existing at the wheels. The brake pressures of the individual wheel brakes can be adjusted or regulated independently of one another. In conjunction with a conventional four-wheeled motor vehicle, this is therefore referred to as a “four-channel regulating actuator suite.” The brake system encompasses, inter alia, a hydraulic assemblage made up of a housing block fitted with pumps and valves, and an electronic control device that actuates those pumps and valves as a function of sensor signals that represent the slip conditions at the individual wheels. Each wheel brake of the vehicle braking system has associated with it a respective brake pressure buildup valve and a respective brake pressure lowering valve for brake pressure regulation.
This primary actuator suite allows the driving state of a vehicle to be stabilized during a braking operation, upon initial movement, or during driving operation, by reducing the brake pressure at wheel brakes of wheels affected by slip. This reduced brake pressure can be generated jointly with the driver or independently of the driver. The primary actuator suite accordingly works in a so-called partly active or fully active mode.
This conventional vehicle braking system furthermore has a second actuator suite or secondary actuator suite in the form of an electromechanical brake booster. This secondary actuator suite is typically connected to the brake master cylinder and serves to enhance driving convenience during normal operation by assisting the driver in building up a brake pressure required for a braking operation. An electromechanical brake booster encompasses for this purpose an electronically controllable actuator that furnishes an external force for actuation of a brake master cylinder. Actuation of the brake master cylinder can be accomplished solely by the external force of the secondary actuator suite, or by a combination of that external force with a muscle force furnished by the driver.
The first and second actuator suite, or primary and secondary actuator suite, consequently constitute two mutually redundant systems for generating and modulating a brake pressure in a vehicle braking system, that brake pressure modulation being capable of being carried out respectively with or without the driver's participation. The two actuator suites thus meet an essential basic prerequisite for implementing and carrying out a partly or fully automated driving mode. Because the driver merely performs a monitoring function during such an automated driving mode, particularly stringent requirements exist in terms of the fail-safe performance of such electronically pressure-controllable vehicle braking systems, those requirements being met by the availability of the two actuator suites explained above.
In contrast to the primary actuator suite, however, by actuation of the brake master cylinder the secondary actuator suite is able to supply only a uniform brake pressure to all the wheel brakes of the vehicle braking system connected to that cylinder, or only to modulate that brake pressure uniformly. This functionality is referred to in technical language as a “single-channel” control actuator suite. A secondary actuator suite designed with a single channel is nevertheless sufficient to decelerate a vehicle to a complete stop, while maintaining its directional stability, in the event of a malfunction of the primary actuator suite.
Minimum requirements for longitudinal or directional stabilization of the vehicle are adherence to a locking sequence, i.e., a brake pressure buildup, in such a way that the wheel brakes of the front axle lock before the wheel brakes of the rear axle; furthermore maintaining steerability of the vehicle and consequently ensuring a maximum locking time for the vehicle wheels and a capability for active or driver-independent buildup of a brake pressure.
The result in particular of the aforementioned criterion of limiting the locking time of the wheels is that the maximum achievable deceleration values for the vehicle depend on the braking performance that can be converted by the wheel brakes of the rear axle. This convertible rear axle braking performance is comparatively low as a result of the dynamic axle load shift toward the front axle that takes place, due to inertia, in the course of a braking operation. Because an increase in axle load at the front axle is necessarily accompanied by a reduction in the axle load at the rear axle, the latter's wheels tend to lock considerably earlier, or at lower brake pressures, than the front wheels which are comparatively more heavily loaded.
Given the property of the secondary actuator suite, as explained, of being able to act upon all the existing wheel brakes only with a uniform brake pressure, in combination with a low brake pressure that can be converted by the wheel brakes of the rear axle without a risk of locking the associated wheels, the result in the case of a braking operation in which brake pressure is applied by the secondary actuator suite, because a malfunction has occurred in the primary actuator suite, is the disadvantage that the total braking performance that can be implemented by the vehicle is relatively low, consequently resulting in a relatively long braking distance for the vehicle. This has a particularly negative effect in vehicles in which the dynamic axle load shift toward the front axle in the context of a braking operation is particularly large.
To avoid this disadvantage, it is furthermore conventional to equip electronically pressure-controllable vehicle braking systems of the species with a further actuator suite, referred to below as a “third actuator suite,” at the wheel brakes of the rear axle.
This third actuator suite is a further electronically controllable unit that is activated in the event of a fault in the primary actuator suite and adapts the brake pressure, furnished by the secondary actuator suite at the wheel brakes of the rear axle, to the reduced weight load at the rear axle which occurs in the context of that braking operation. It is thereby possible, by corresponding control application to the actuator of the secondary actuator suite, for the latter to furnish a brake pressure that can be entirely converted into braking performance only by the wheel brakes of the front axle, which is more heavily loaded in the context of a braking operation, by the fact that the third actuator suite interrupts a further rise in the brake pressure at the wheel brakes of the rear axle as soon as that brake pressure approaches a threshold value at which that brake pressure can still be entirely converted into braking performance. In other words, thanks to the third actuator suite the brake pressure level of the vehicle braking system is no longer limited, as explained above, to the lower brake pressure level convertible by the wheel brakes of the rear axle, and it furthermore becomes possible to utilize in its entirety the braking performance that can be transferred by the wheel brakes of the front axle. The overall result is that in this fashion, a higher total braking performance for the vehicle braking system, and consequently a considerably shorter braking operation and braking distance, can be achieved.